[OpenBIOS] Incorrect implementation of b?branch?

[Stefan Reinauer]

On 21.02.2009 13:20 Uhr, Mark Cave-Ayland wrote:
> Hi everyone,
>
> I've been spending some time over the past few weeks trying to get a
> Sparc64/Solaris disk image to boot under qemu, and as far as I can
> tell, the first reason that the code terminates incorrectly is because
> b?branch doesn't work.
>
> This can be demonstrated fairly easily using the following couple of
> Fcode programs. Note that while I'm working on Sparc64, I see the same
> issue on x86 OpenBIOS too.
>
>
> Example 1: b?branch with -1 on stack: ignore branch & abort
>
> CC        \ offset16
> 10        \ b(lit)
> FF FF FF FF    \ (-1)
> 14 00 04    \ b?branch
> 02 16        \ if false, abort
> 9D        \ if true, display stack
> 00        \ end
>
> 0 > true to ?fcode-verbose  ok
> 0 > here CC10FFFFFFFF1400 /n allot over !  ok
> 1 > here 0402169D00000000 /n allot over ! drop  ok
> 1 > 1 byte-load
> byte-load: evaluating fcode at 0xffd9d128
> fcode-table at 0xffdb7f90
> offset16 [ 0xcc ]
> b(lit) [ 0x10 ]
> b?branch [ 0x14 ]
> abort [ 0x216 ]
>
> byte-load: exception caught!
>  ok
>
>
> Example 2: b?branch with 0 on stack: skip abort, display stack
>
> CC        \ offset16
> 10        \ b(lit)
> 00 00 00 00    \ (0)
> 14 00 04    \ b?branch
> 02 16        \ if false, abort
> 9D        \ if true, display stack
> 00        \ end
>
> 0 > true to ?fcode-verbose  ok
> 0 > here CC10000000001400 /n allot over !  ok
> 1 > here 0402169D00000000 /n allot over ! drop  ok
> 1 > 1 byte-load
> byte-load: evaluating fcode at 0xffd9d128
> fcode-table at 0xffdb7f90
> offset16 [ 0xcc ]
> b(lit) [ 0x10 ]
> b?branch [ 0x14 ]
> abort [ 0x216 ]
>
> byte-load: exception caught!
>  ok
>
>
> Okay, so did anyone notice the difference?! This led me to look into
> the definition of b?branch in forth/device/fcode.fs:
>
>
> \ b?branch ( continue? -- )
> \   Conditional branch FCode. Followed by FCode-offset.
>
> : b?branch
>   ['] do?branch ,
>   fcode-offset 0< if \ if we jump backwards, we can forsee where it goes
>     resolve-dest
>   else
>     here
>     0 ,
>   then
>   ; immediate
>
>
> By adding some extra debugging into b?branch, I can see that for the
> above Fcode, do?branch is *never* called, although it correct takes
> the "else" branch listed above.
>
> I'm still reasonably new to Forth, but AIUI, all the code above does
> is allocate one cell to store the xt of do?branch, and in the case of
> a forward branch, push the here location onto the stack and then store
> 0 into another allocated cell. This agrees with my investigation that
> shows that do?branch is never actually called.
>
> Given that this code is also missing a section that will read and
> discard Fcode-offset-2 bytes for a forward branch, I'm inclined to
> think that this code is just wrong. Can anyone else with any
> experience with this code comment upon this?

I think you indeed hit a bug. b?branch in openbios only works in word
definitions, not in interpretion mode. It needs similar handling as
setup-tmp-comp/execute-tmp-comp in the definition of "if" in bootstrap.fs.

In addition to that your example contains non-valid code. A b?branch
always needs to point behind a b(>resolve) as in the below example.

here
cc c,                   \ offset16
\ a4 c,                 \ -1
a5 c,                   \ 0
14 c, 00 c, 05 c,       \ b?branch
02 c, 16 c,             \ abort
b2 c,                   \ b(>resolve)
9f c,                   \ .s
00 c,                   \ end0
1 byte-load


Forth equivalent:
\ -1
0
if abort then
.s
end0

Stefan


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